1. Field of the Invention
The present invention relates to a developing device included in an image forming apparatus, such as a copier, a facsimile, and a printer, and further relates to an image forming apparatus including the developing device. In particular, the present invention relates to toner concentration control in a developing device that uses a two-component developer containing a magnetic carrier and a toner to develop an electrostatic latent image on an image bearing member.
2. Description of the Related Art
Typical dry-toner developing methods for electrophotographic image forming apparatuses use a one-component developing method or a two-component developing method. The one-component developing method uses no carrier, while the two-component developing method uses a two-component developer in which a magnetic carrier is used to charge a non-magnetic toner. In the two-component developing method, an electrostatic latent image on an image bearing member (photosensitive member) is developed by a magnetic brush of a toner and a carrier formed on a developing roller.
The one-component developing method is suitable for forming high-quality images in that an electrostatic latent image on the image bearing member is not disturbed by the magnetic brush. However, since toner is charged by a charging roller and the thickness of a toner layer on the developing roller is controlled by an elastic control blade, toner additives adhere to the charging roller. This degrades the charging capability of the charging roller and makes it difficult to maintain a constant amount of charged toner. Moreover, adhesion of toner to the control blade may cause an uneven layer to be formed and may result in defective images.
Color printing, which involves superimposition of different colors, requires color toners to be transparent. This means that the color toners need to be non-magnetic toners, since known magnetic toners are not sufficiently transparent. Therefore, full-color image forming apparatuses typically adopt the two-component developing method in which carriers are used to charge and convey toners. The two-component developing method can maintain a constant amount of charged toner over a long time, and is suitable for realizing a longer toner life. However, the two-component developing method may be disadvantageous in that the magnetic brush described above may affect image quality.
As a means for solving the problems described above, in one known developing method, when a developer is moved by a magnetic roller (toner supplying member) onto a developing roller (toner bearing member) that is not in contact with an image bearing member, only a non-magnetic toner is transferred onto the developing roller to form a thin toner layer thereon, with a magnetic carrier remaining on the magnetic roller. Then, an alternating electric field causes the toner to adhere to an electrostatic latent image on the image bearing member.
Toner concentration in a developer may be controlled by adjusting the amount of toner supply depending on a toner concentration detected by a magnetic permeability sensor, etc., or it may be controlled by forming a patch image (reference toner image) to adjust the amount of toner supply on the basis of a detected density of the patch image. However, during continuous printing, image density of a patch image decreases with time. Therefore, if the amount of toner supply is increased in response to the decrease in image density, the toner may be supplied excessively and an image defect, such as fogging, may result.
One known method uses a toner concentration detector that detects a toner concentration in a developer, and an image density detector that optically detects a density of a test pattern (patch image) formed on an image bearing member. In this method, when a toner concentration detected by the toner concentration detector reaches a predetermined upper limit of a toner concentration, toner supply is stopped. Thus, even when a charge potential, exposure, and developer on a photosensitive drum change with time, it is possible to prevent changes in image density.
However, in the method described above, toner concentration control is performed by switching from control based on the result of image density detection to control based on the result of toner concentration detection, according to the size of the patch image. Thus, this method does not prevent image density of a reference toner image from decreasing over time during continuous printing.
During continuous printing, there is a tendency in which toner gradually adheres to a developing roller surface and an electric charge accumulates on the developing roller surface. When an electric charge gradually accumulates on the developing roller surface over time until saturation is reached, the potential of the developing roller surface increases and a direct-current potential difference (effective potential) between the developing roller and a magnetic roller decreases.
Thus, in the two-component developing method described above, the amount of toner supplied from the magnetic roller to the developing roller during continuous printing may decrease with time, and the image density may decrease accordingly. This results in a decrease in the amount of toner supplied to a patch image, affects the detected image density of the patch image, and further affects the toner concentration correction.
When, on the basis of a patch image, a toner concentration correction is performed between printing operations and between predetermined points during continuous printing, the density of the patch image in the toner concentration correction performed during continuous printing is lower than that in the toner concentration correction performed between printing operations. This is because an image density of the patch image decreases with time during continuous printing, which will result in an error in toner concentration correction.